Yeah, Lead sheeting from old radiology rooms is like stiff, thick, tin foil it's so soft. You have to keep an eye out for old doctor-office buildings that are going to be torn down, as they will have the sheeting, as well as older hospitals doing upgrades.
Sailboat ballast is good BUT it's often in huge chunks, so it's soft, but you need something more like a wood saw not a hack saw to break it down into chunks to fit into your melting pot. The fine teeth of a standard hacksaw blade get clogged with the soft lead....or you might use a torch and suspend the lead over a pot and melt it into the pot....
LD